package linklist;

import queue.Queue;

/*
 * 使用带有尾指针的单向链表实现队列
 *
 * 带有尾指针的链表在头和尾添加元素都简单O(1)
 * 但是在链尾删除元素为O(n)
 * 因此实现队列需要在链表尾添加元素(队列的尾), 链表头删除元素(队列的头), 这样队列的入队出队都为O(1)
 */
public class LinkedList3Queue<E> implements Queue<E> {
    private class Node {
        public E e;
        public Node next;

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }
        public Node(E e) {
            this(e, null);
        }

        public Node() {
            this(null, null);
        }

        @Override
        public String toString() {
            return e.toString();
        }
    }

    private Node head, tail;
    private int size;

    public LinkedList3Queue() {
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public int getSize() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void enqueue(E e) {
        if (tail == null) {
            tail = new Node(e);
            head = tail;
        } else {
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E dequeue() {
        if(isEmpty())
            throw new IllegalArgumentException("Cannot dequeue from an empty queue.");

        Node ret = head;
        head = head.next;
        ret.next = null;
        if (head == null)
            tail = null;
        size --;
        return ret.e;
    }

    @Override
    public E getFront() {
        if(isEmpty())
            throw new IllegalArgumentException("Queue is empty.");
        return head.e;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");

        Node cur = head;
        while (cur != null) {
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");

        return res.toString();
    }

    public static void main(String[] args) {
        LinkedList3Queue<Integer> queue = new LinkedList3Queue<>();
        for(int i = 0 ; i < 10 ; i ++){
            queue.enqueue(i);
            System.out.println(queue);

            if(i % 3 == 2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}
